One of my friends has a '91 XJ with ~120k miles. It's been sitting for a few months now due to the serpentine belt getting all chewed up when the engine runs. He said a mechanic came over to look at it, and the mechanic said the crankshaft was bent. The motor ran fine when it was parked, the only problem was the belt. I can't imagine how a crankshaft could bend inside the engine, and if it could, I'd think the engine would run poorly. So, my question is, can a crankshaft really bend while it's still in the engine? I really can't figure out how it would.

Any other ideas as to what's causing the belt problem? The only pulley the belt jumps a groove on is the crank pulley, but I can't tell for sure if the crank pulley is the problem.

I'd bet the crank pulley is warped, wobbling, or damaged in some way which is causing your belt issues. It is possible that a harmonic balancer can be put on in a way that causes it to wobble (the hammer-it-on method) which could also cause the crank pulley to wobble and result in belt problems. Normally you can crank and engine and closely watch the crank pulley spinning as the engine idles. Any strange problems should be obvious.

Perhaps the mechanic is saying the snout of the crank is bent and that's causing the balancer & pulley to wobble, which is shredding the belt. I don't see how that could happen though through normal driving use. Have you ever had the crank replaced?

Usually a bent crank is the result of something destructive that has happened and pistons and/or rods have ejected from the engine, in which case a shredded belt is the least of your concerns!

Sorry I can't be of more help, but it's impossible to say without seeing the engine in question. Good luck.

To answer your first question, yes it is very possible for a crank to bend inside the engine. Though it will definitely NOT still run fine, haha. If its running fine still, it is probably just a pulley problem thats tearing through belts. It could just be a chunk missing from a pulley that you cant see, maybe because the piece that is missing is under the belt? Therefore you cant see it eating away at your belt. If you really cant figure out the problem, just start replacing all the pulleys, process of elimination.

One of my friends has a '91 XJ with ~120k miles. It's been sitting for a few months now due to the serpentine belt getting all chewed up when the engine runs. He said a mechanic came over to look at it, and the mechanic said the crankshaft was bent. The motor ran fine when it was parked, the only problem was the belt. I can't imagine how a crankshaft could bend inside the engine, and if it could, I'd think the engine would run poorly. So, my question is, can a crankshaft really bend while it's still in the engine? I really can't figure out how it would.

Any other ideas as to what's causing the belt problem? The only pulley the belt jumps a groove on is the crank pulley, but I can't tell for sure if the crank pulley is the problem.

I can almost guarantee it is NOT a bent crankshaft, it is the harmonic balancer coming apart. If you look at the HB, it has rubber sandwiched between the hub of the unit and the pulley. As it gets old, the rubber deteriorates and the HB will separate and eventually launch the pulley into tender parts of your vehicle (or someone else's) and cause ALOT of damage.

I replaced mine on my '88 not too long ago, I think it cost me $50 or $60 from Auto Zone. What ever you do, get a new one and not a used one!

Thanks to all who replied. I'll pass on that information to my friend, and see if he'll be willing to cut into his cigarette budget in order to fix his car. Prolly not, knowing him.

I'm guessing a harmonic balancer isn't all that hard to replace? Just need the proper puller/installer tool, right? Are they keyed on to the crankshaft, like small block Chevy motors?

Yup, you need a puller, a socket that will fit the nut in the center and a wrench or two to loosen the belt. It's not a hard job, similar to alot of motors out there. When you put the new one on, use the center bolt to fully press the pulley onto the crank. You can get it started with some gentle persuasion from a hammer (just don't beat it to death), then use the bolt to press it on the rest of the way. Also, while you're in there, replace the seal. If you don't, it will leak for sure.

I am going to have to agree that the crank is not bent. The motor would come unglued otherwise. When you tear into the belts and pullys, not sure how your wrangler is but, just in case the tension adjust is on the power stearing pump, not on an idler pully like it should be. Probably obvious but just in case. Good luck

There have been a couple of articles I have read that would lead me to believe it could aslo be a bad bearing. I can't find the article, but will look for it. It talks about there being some end play in the crank. The crank would pull reward when running and the harmonic balancer would rub into the timing cover.

If he pull sthe balancer and all is well, then disregaurd this, but if the balancer is good and it still happens, that might be somethng to look at.

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I am going to have to agree that the crank is not bent. The motor would come unglued otherwise. When you tear into the belts and pullys, not sure how your wrangler is but, just in case the tension adjust is on the power stearing pump, not on an idler pully like it should be. Probably obvious but just in case. Good luck

Well, my Wrangler has V-belts, so it is rather different. I've replaced the serpentine belts before on Cherokees, so I know the power steering pump is what gives the belts the tension. I wish I had a similar thing on my Wrangler, it sure beats using the big screwdriver technique to get the belts tight.

Balloo, I don't think it's the crank going backwards, it looks more to be the belt going backwards. I'll see if I can feel any free play in the crank, though.

How would I go about replacing the seal? Is it something that'll just pop out when I have the balancer off, or is it something I would need to drop the oil pan for?